1. Field of the Invention
The present invention relates to drive capstans for magnetic tape transports, and more particularly to a low inertia capstan for use in magnetic tape transports of the digital type designed for intermittent, bidirectional operation such as in a data processing environment.
2. History of the Prior Art
As data processing and similar operations have become faster and more sophisticated, increasingly greater demands have been placed on associated magnetic tape equipment. Such equipment may include one or more tape transports, each having one or more capstans which continuously or selectively engage magnetic tape and which must be capable of accelerating the tape from rest to nominal speed within an extremely short period of time, maintaining the nominal speed for as long as necessary, and then either decelerating the tape to rest or reversing the direction of drive of the tape, again in an extremely short time.
The relatively rapid acceleration and deceleration required in systems of the type described is accomplished in part by use of capstans of relatively light weight so as to minimize the resulting inertia of the capstan. At the same time the capstan must have sufficient strength and sturdiness to be capable of supporting the tape under considerable tension without distortion and to permit handling of the capstan without physical damage. Most capstans are made of a lightweight metal such as aluminum in order to provide the necessary strength. However since almost all metals including aluminum have a relatively high elastic modulus, a metal capstan may be required to have considerable mass to maintain the required structural integrity while shipping, assembling and otherwise handling the capstan. If the capstan is made too thin or is overloaded so as to exceed its elastic limit, the capstan will become bent or otherwise permanently distorted and thereby rendered useless. In an effort to achieve low inertia by minimizing the amount of material used for construction, the strength of a conventional capstan is often reduced to such an extent that the mere inadvertent dropping of such a capstan on a floor or other hard surface is often enough to damage the capstan beyond repair because of the close tolerances involved.
In an effort to make capstans relatively strong and rigid, metals of substantial thickness have been used. However this adversely affects the capstan in terms of mass, inertia and other factors. Increasing the thickness of the metal parts greatly increases the resulting mass and inertia of the capstan, particularly if the cylindrical outer rim is increased in thickness as is often required. Capstans of relatively large inertia are difficult to accelerate and decelerate within allowed times and further decrease system performance by increasing power consumption, increasing the amount of heat which must be dissipated, and requiring a larger capstan drive motor with its own increased inertia. Other disadvantages which arise out of the demand for capstans of substantial sturdiness include the concomitant decrease in the number or size of holes in the outer rim which are permitted in order to achieve pneumatic handling of the tape relative to the capstan.
These circumferential holes are frequently provided in the capstan outer rim to permit communication of a vacuum or other source of reduced pressure in the vicinity of the capstan to the tape and thereby enhance the engagement of the capstan with the tape. Such holes may also function to communicate a source of high pressure in the vicinity of the capstan to the region between the capstan outer surface and the tape, thereby producing an air bearing between the capstan and the tape during certain modes of operation. The strength and rigidity requirements of the capstan frequently dictate that such holes in the outer rim be small and far enough apart so as to prevent distortion of the capstan, particularly beyond its elastic limit. At the same time however the holes of relatively small size and spaced a considerable distance from one another greatly decrease the communication of both high and low pressure sources through the capstan outer rim.
Accordingly it would be advantageous to provide a capstan having a considerably lower inertia than the metal capstans now in use while at the same time having the necessary strength for driving the tape under conditions of considerable tension without resultant damage or destruction of the capstan. Such capstan should desirably have an outer rim, a substantial portion of the surface area of which may comprise apertures or grooves combined with apertures so as to enhance pneumatic holddown of the tape or the formation of an air bearing therewith without at the same time sacrificing the ability of the capstan to support the tape under substantial tension. Such capstan should also desirably be so constructed so as to withstand occasional shipping, assembly and other handling abuses such as inadvertent dropping thereof without damage or destruction and should be relatively easy to fabricate.